1. Field of the Invention
This invention relates to a method and apparatus for the separation of a catalyst phase from a gas suspension phase in a fluidized catalytic cracking unit (FCC). More particularly, it relates to an improved method and apparatus for separating the catalyst phase from the gas suspension phase, as the gas suspension phase is discharged from a riser converion zone outlet, to minimize or substantially eliminate post-riser conversion zone cracking.
2. Discussion of the Prior Art
The field of catalytic cracking, particularly fluid catalytic cracking, has undergone significant development improvements due primarily to advances in catalyst technology and product distribution obtained therefrom. With the advent of high activity catalysts and particularly crystalline zeolite cracking catalysts, new areas of operating technology have been encountered requiring refinements in processing techniques to take advantage of the high catalyst activity, selectivity and operating sensitivity.
By way of background, the hydrocarbon conversion catalyst usually employed in an FCC installation is preferably a high activity crystalline zeolite catalyst of a fluidizable particle size. The catalyst is transferred in suspended or dispersed phase condition generally upwardly through one or more riser conversion zones (FCC cracking zones) providing a hydrocarbon residence time in each conversion zone in the range of 0.5 to about 10 seconds, and usually less than about 8 seconds. High temperature riser hydrocarbon conversions, occurring at temperatures of least 1000.degree. F. or higher and at 0.5 to 4 seconds hydrocarbon residence time in contact with the catalyst in the riser, are desirable for some operations before initiating separation of vaporous hydrocarbon product materials from the catalyst. Rapid separation of catalyst from hydrocarbons discharged from a riser conversion zone is particularly desirable for restricting hydrocarbon conversion time. During the hydrocarbon conversion step, carbonaceous deposits accumulate on the catalyst particles and the particles entrain hydrocarbon vapors upon removal from the hydrocarbon conversion step. The entrained hydrocarbons are subjected to further contact with the catalyst until they are removed from the catalyst by mechanical means and/or stripping gas in a separate catalyst stripping zone. Hydrocarbon conversion products separated from the catalyst and stripped materials are combined and passed to a product fractionation step. Stripped catalyst containing deactivating amounts of carbonaceous material, hereinafter referred to as coke, is then passed to a catalyst regeneration operation.
Of particular interest has been the development of methods and systems for separating catalyst particles from a gas suspension phase containing catalyst particles and vaporous hydrocarbon product materials, particularly the separation of high activity crystalline zeolite cracking catalysts, under more efficient separating conditions so as to reduce overcracking of hydrocarbon conversion products and promote the recovery of desired products of a hydrocarbon conversion operation. Cyclonic equipment is now typically used for efficient separation of fluidizable catalyst particles from the gas suspension phase. However, present day cyclonic equipment often permits an undesirable extended residence time of the product vapor within a large reactor vessel. This extended residence time reduces the desired product yield by as much as 4 percent through non-selective thermal cracking. Recent developments in this art have been concerned with the rapid separation and recovery of entrained catalyst particles from the gas suspension phase.
Various processes and mechanical means have been employed heretofore to effect rapid separation of the catalyst phase from the hydrocarbon phase at the termination of the riser cracking zone, to minimize contact time of the catalyst with cracked hydrocarbons. Several of these are discussed below.
Cartmell, U.S. Pat. No. 3,661,799, discloses a process for catalytic conversion of petroleum feedstocks wherein the fluidized mixture of the cracking catalyst and cracked feedstock leaves a vertically-disposed reactor section and enters a cyclone separator, placed in a separate stripper vessel, through a conduit. The conduit contains an annulus which allows an inert stripping gas and associated stripped vapors to pass into the cyclone separator.
Anderson et al, U.S. Pat. No. 4,043,899, discloses a method for rapid separation of a product suspension, comprising fluidized catalyst particles and the vaporous hydrocarbon product phase, by discharging the entire suspension directly from the riser conversion zone into a cyclone separation zone. The cyclone is modified to include a separate cyclonic stripping of the catalyst separated from the hydrocarbon vapors. In the method of Anderson et al, the cyclone separator is modified to include an additional downwardly extending section comprising a lower cyclone stage. In this arrangement, catalyst separated from the gasiform material in the upper stage, slides along a downwardly sloping baffle to the lower cyclone where stripping steam is introduced to further separate entrained hydrocarbon products from the catalyst recovered from the upper cyclone. The steam and the stripped hydrocarbons are passed from the lower cyclone through a concentric pipe where they are combined with the hydrocarbon vapors separated in the upper cyclone. The separated and stripped catalyst is collected and passes from the cyclone separator by conventional means through a dipleg. This process requires that the entire volume of catalyst, gasiform phase and stripping steam pass through the cyclone separator, which means that this equipment must be designed to efficiently handle not only a large vapor volume, but also a large quantity of solid particles.
Myers et al, U.S. Pat. No. 4,070,159, provides a separation means whereby the bulk of catalyst solids is discharged directly into a settling chamber without passing through a cyclone separator. In this apparatus, the discharge end of the riser conversion zone is in open communication with the disengaging chamber such that the catalyst discharges from the riser in a vertical direction into the disengaging chamber which is otherwise essentially closed to the flow of gases. The cyclone separation system is in open communication with the riser conversion zone by means of a port located upstream from, but near, the discharge end of the riser conversion zone. A deflector cone mounted directly above the terminus of the riser causes the catalyst to be directed in a downward path so as to prevent the catalyst from abrading the upper end of the disengaging vessel. The cyclone separator is of the usual configuration employed in a catalytic cracking unit to separate entrained ctalyst particles from the cracked hydrocarbon products so that the catalyst passes through the dipleg of the cyclone to the body of the catalyst in the lower section of the disengaging vessel, and the vaporous phase is directed from this vessel to a conventional fractionation unit. There is essentially no net flow of gases within the disengaging vessel beyond that resulting from a moderate amount of steam introduced to strip the catalyst residing in the bottom of the disengaging vessel.
Haddad et al, U.S. Pat. No. 4,219,407, discloses the separation of the catalyst from the gasiform cracked products in a fashion which permits effective steam stripping of the catalyst. The suspension of catalyst and gasiform material is discharged from the riser conversion zone outwardly through radially extending passageways, or arms, which terminate in a downward direction. Catalyst stripping zones, or strippers, are located beneath the terminus of each of the radially extending passageways. Each stripper consists of a vertical chamber open at the top and the bottom with downwardly sloping baffles located within the chamber so as to cause the catalyst to flow in a discontinuous manner countercurrently to upwardly flowing stripping stream introduced at the lower end of the stripping chamber. The radially extending arms are each provided with a curved inner surface and confining sidewalls to impart a cyclonic concentration of catalyst particles promoting a forced separation thereof from the hydrocarbon vapors. The separation of the catalyst from the vapors is basically achieved through rapid changes in the direction of flow of the catalyst and the vapors. Thus the cyclonic collection and concentration of catalyst particles is used to reverse the flow of separated catalyst such that it concentrates as a downwardly confined stream which discharges generally downwardly and into the open upper end of the catalyst stripping chamber. A vapor disengaging space is provided between the discharge end of the radially extending arms and the top of the catalyst strippers to promote the rapid removal of separated vapors from the catalyst. The separated vapors pass upwardly through the disengaging vessel to the open inlet of a cyclone separator which removes entrained catalyst from the gasiform material for return through a dipleg to the body of steam stripped catalyst while the separated vaporous material passes to a fractionation unit. The hydrocarbon product, as it passes within the disengaging vessel from the discharge of the radially extending arms to the cyclone separator, travels in a random fashion and is exposed to catalytic reaction temperatures which may cause undesirable side reactions and thermal cracking before these vapors enter a quench zone in the main fractionator of the fluid cracking unit.
Haddad et al, in U.S. Pat. No. 4,404,095, filed July 22, 1982, the disclosure of which is incorporated herein by reference, disclose an FCC reactor comprising a riser with radially extending sidearms as the first catalyst-hydrocarbon separation means. The sidearms force the suspension of the catalyst and the hydrocarbons to suddenly change the direction of flow from the vertical to the horizontal thereby forcing preliminary separation of the gaseous hydrocarbons from the solid catalyst particles. The catalyst particles fall in a downward direction, to a stripping means, while the hydrocarbons, with some entrained catalyst particles, proceed to a secondary separation means, such as a cyclone. The sidearms and the secondary separation means are enclosed by a vertical conduit to prevent random uncontrolled thermal cracking of the hydrocarbons. However, the vertical conduit provided to send hydrocarbons from the side arms to the secondary separation means may not accommodate radial and longitudinal thermal expansion of the separation means.